This invention relates to marine seismic streamers, and to methods of manufacturing such streamers.
This invention relates to marine seismic streamers, and to methods of manufacturing such streamers.
In order to perform a 3D marine seismic survey, an array of marine seismic streamers, each typically several thousand metres long and containing a large number of hydrophones and associated electronic equipment distributed along its length, is towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitised and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
A typical marine seismic streamer is made up of a large number of similar 100 metre streamer sections connected end-to-end, each section comprising a substantially cylindrical outer skin containing a pair of longitudinally extending strength members, typically Keviar ropes, to bear the towing forces. The hydrophones are substantially uniformly distributed along the length of the streamer section, and are interspersed with cylindrical spacers and foam elements which are mounted on the strength members, the foam elements being saturated in use with kerosene or a similar fluid to render the streamer section substantially neutrally buoyant. A streamer section of this general type, and more particularly its method of manufacture, are described in our co-pending PCT Patent Application No. PCT/IB99/00624 (WO99/), filed Apr. 15, 1998.
The streamer also includes electronics modules (or xe2x80x9cbubblesxe2x80x9d) containing circuitry for digitising the reflected signals detected by the hydrophones and transmitting the digitised signals to the seismic survey vessels, these modules typically being connected between adjacent streamer sections.
While marine seismic streamers made up of streamer sections of this general type provide good overall performance, there is a continuous need for improvements in areas such as acoustic performance, diameter (reduced diameter gives reduced drag and easier handling), and robustness.
It is an object of the present invention in one of its aspects to provide a marine seismic streamer exhibiting improvements over the known type of streamer in at least some of these areas. It is a further object of the invention in another of its aspects to provide a method of manufacturing such an improved streamer.
According to one aspect of the present invention, there is provided a seismic streamer comprising an elongate substantially solid core, at least one longitudinally extending strength member and a plurality of hydrophones embedded in said core, a plastics outer skin surrounding said core and defining therearound an annular space, and plastics foam material adapted to be substantially saturated with liquid and substantially filling said annular space.
In a preferred embodiment of the invention, said hydrophones are encapsulated in capsules having at least one opening communicating with said annular space.
The streamer may include a plurality of foam buoyancy elements, typically made from a skinned plastics foam material such as skinned polypropylene foam, embedded in said core between said encapsulated hydrophones, and may further include a plurality of electronics modules each containing electronic circuitry for processing the signals produced by a plurality of said hydrophones, said electronics modules being embedded in said core between said hydrophones, along with electrical conductors for connecting the hydrophones to the electronics modules, electrical conductors for providing power to the electronics modules, electrical conductors for connecting the electronics modules to respective electro-optical conversion and multiplexing units connected in the streamer at intervals therealong, and optical fibres for interconnecting the electro-optical conversion and multiplexing units.
According to another aspect of the invention, there is provided a method of making a marine seismic streamer comprising an elongate substantially solid core, and at least one longitudinally extending strength member and a plurality of hydrophones embedded in said core, the method comprising the steps of:
forming a subassembly comprising said at least one strength member having said hydrophones secured thereto at intervals therealong;
drawing said subassembly through a plastic tube; and
injecting plastics material or gel into said tube to substantially fill the rest of the interior of the tube.
In a preferred implementation of this aspect of the invention, the subassembly preferably comprises two parallel strength members, and the method further includes securing a plurality of foam buoyancy elements in said subassembly between the hydrophones, securing a plurality of electronics modules each containing electronic circuitry for processing the signals produced by a plurality of the hydrophones in said subassembly between the hydrophones, and securing electrical conductors connecting the hydrophones to the electronics modules, electrical conductors for providing power to the electronics modules, and optical fibres in said subassembly.
According to yet another aspect of the invention, there is provided a seismic streamer comprising an elongate plastics tube, at least one elongate strength member disposed within the tube and extending longitudinally thereof, a plurality of buoyancy elements which engage the strength member at intervals therealong, and a plurality of hydrophones longitudinally spaced apart within the tube and disposed between the buoyancy elements, the tube containing a substantially solid plastics or gel filler material around the hydrophones and the buoyancy elements.
According to a still further aspect of the invention, there is provided a method of making a seismic streamer, the method comprising the steps of:
forming a subassembly comprising at least one elongate strength member having buoyancy elements secured thereto at intervals therealong and hydrophones disposed between some of the buoyancy elements;
drawing said subassembly through a plastic tube; and
injecting plastics material or gel into said tube to substantially fill the rest of the interior of the tube.